1. Field Of The Invention
The present invention relates to portable power tools of the hammer type. More particularly, the invention relates to power tools of this type wherein a striker, usually in the form of a piston, is reciprocated by power means for imparting successive impact blows to the end of a tool shank element.
2. The Prior Art
Many prior art tools of the type under consideration employ pneumatic means for reciprocating the striker piston. In this regard, reference is made to Naslund U.S. Pat. No. 3,114,423, assigned to the assignee of the present invention; this patent discloses such a pneumatically operated hammer device. According to the tool shown in the Naslund patent, a first piston is reciprocated within a barrel by the usual crankshaft and connecting rod assembly. A second piston, i.e., a striker piston, is freely mounted in the barrel and is pneumatically reciprocated as a consequence of the reciprocation of the first mentioned piston.
The use of pneumatic means for reciprocating the striker permits the development of high impact blows, and minimizes the development of rebound shocks due to the pneumatic cushion between the aforementioned pistons. These pneumatic systems reciprocate the striker or free piston in a manner such that its velocity in the hammer-direction, i.e., the direction toward the tool shank element, is greater than the velocity of the striker in the opposite direction, i.e., the returndirection. Or stated another way, the energy of such a pneumatically operated striker piston is greater in its hammerdirection than in its return-direction. The feature of having the piston returned at a lower velocity, and consequently developing less energy, is of course advantageous in that the rebound shocks or forces are minimized thereby contributing to operator comfort and increased tool life.
Such power hammers suffer from certain disadvantages. In this regard, problems are often encountered in attempting to maintain the necessary air seal between the pistons and the cylinder, especially for long periods of time and under adverse conditions to which tools of this type are subjected. Because of this seal problem, it is often difficult to start pneumatic tools in cold weather. Also, it is sometimes difficult to employ a completely sealed lubrication system with certain types of pneumatic tools.